1. Field of Invention
This invention relates to antennas and more specifically, to an antenna circuit and matching technique for optimizing small loop antenna performance.
2. Description of the Related Art
Small loop antennas are commonly used in many applications because of their sharply defined radiation pattern, small size and performance characteristics. For example, a cordless keyboard and receiver can be implemented with small loop antennas. When designing a loop antenna, one must consider the effect of certain parasitic elements. In particular, ohmic losses and the capacitive reactances can have the effect of lowering the performance of the antenna for many reasons. Specifically, the ohmic losses can directly reduce the antenna maximum efficiency as measured by the equation: eff=Rr/R1, where Rr is the radiation resistance and R1 is the ohmic loss of the antenna. As can be seen, the greater the ohmic loss of the antenna (R1), the lower the antenna efficiency.
Parasitic capacitances, on the other hand, can effectively create reactive pathways between the loop segments of a loop antenna, or between the turns of a multiple loop antenna. The result is that a portion of the performance current delivered to the antenna is directed between the loop segments or turns that comprise the conductor of the antenna instead of flowing along the conductor of the antenna for maximum magnetic flux generation. Thus, optimal radiation is not achieved. In addition to these ohmic and capacitive losses, the self-resonant frequency of the loop antenna may be lower than the actual desired operating frequency. Such a situation can also lead to significant losses as well as require complicated compensation techniques.
Another less known parasitic of the loop antenna is its capability to generate reactive voltages that are associated with the conductor surface of the antenna. These reactive voltages give life to capacitive leakage currents to surrounding environment conductors typically grounded. These capacitive leakage currents to other environments particularly occur at RF frequencies, and effectively create a capacitive radiating element or capacitive antenna. The radiating pattern of this parasitic capacitive antenna then interacts with the radiating pattern of the small loop antenna and potentially degrades the desired antenna performance. To complicate this mater, changes in the surrounding grounded environment conductors cause corresponding changes in the radiating pattern of the capacitive antenna thereby further disturbing the small loop antenna range. Consequently, the reliability of the small loop antenna is subject to variations in the surrounding environment conductors. This is an unacceptable circumstance in many applications because the performance of the antenna is unpredictable and unreliable.
A particular scenario where the problem of capacitive leakage currents is exacerbated is when a radio device is connected to a cable and the cable runs across the field of operation of the small loop antenna. For example, where a receiver unit is connected to a host computer via a cable, and the cable runs across the transmission field of a cordless mouse. The position of the cable, as well as other grounded devices in the vicinity of the small loop antenna, will affect the spurious capacitance of the parasitic capacitive antenna and ultimately change the radiation pattern of the inductive small loop antenna. In short, both antennas, the desired small loop antenna and the unwanted spurious capacitive antenna, will have their radiation patterns summed vectorially. This is undesirable because the vectorial summing contributes to unpredictable antenna performance. Although it is possible that some configurations may actually increase the desired antenna performance, such configurations are merely fortuitous and simply unreliable. Moreover, the opposite result is likely to occur where antenna performance is dramatically reduced. Regardless, the direct consequence is a random variation of the operating range of the small loop antenna. Such a consequence directly limits the application of the antenna because reliability of the antenna is marginal.
Thus, there are many reasons to correctly control and reduce the various parasitic elements of an antenna. One device available for reducing the parasitic capacitive antenna effect to surrounding environment conductors is called a balun (acronym for balance-unbalanced). This device is designed with lumped elements such as transformer devices or striplines, the length of which is a part of the wavelength of the antenna. These balun devices are not always practical, however, because they can be physically large as well as costly. Moreover, such a device does not prevent antenna current from flowing between the loop segments of a loop antenna, and therefore does not optimize magnetic flux generation. Nor does the balun reduce ohmic losses. To the contrary, a balun adds extra losses in the antenna matching circuit, and can require complex tuning procedures.
Shielding the small loop antenna is also a well-known technique that increases the coupling of the loop antenna to the shield ground and thus prevents the electrical field to radiate externally to other grounded devices in the vicinity of the small loop antenna system. However, this solution is not practical for printed circuit board-type loop antennas because of the physical layout of the antenna on the printed circuit board. This technique is therefore materially limited in its application. Moreover, shielding tends to increase capacitive losses of the small loop antenna reducing its effective field of performance.
Therefore, what is needed is an antenna circuit and matching technique for balancing a loop antenna resulting in canceling the effects of the parasitic elements of the antenna. This technique must be usable for very small antennas including printed circuit board (PCB) applications, and must not require the addition of bulky components. The resulting antenna must be balanced about ground, and have a negligible reactive voltage difference between corresponding points of adjacent turns of the antenna. Moreover, the antenna must be immune to environment conditions, and must provide reliable performance at a reasonably low cost.
Accordingly, the present invention provides an antenna circuit that has an average reactive voltage of substantially 0 volts and is therefore balanced about ground. Additionally, for an antenna that has multiple turns, the reactive voltage difference between corresponding points of the adjacent turns is also substantially 0 volts. The present invention also provides an antenna matching technique that produces an antenna that has an average reactive voltage of 0 volts, and a negligible difference between corresponding points of the adjacent turns of the antenna loop. The antenna matching technique cancels the reactive voltage of the antenna conductor inside the antenna rather than canceling the reactive voltage at the antenna ends by appending a matching circuit.
Specifically, serial tuning capacitors are inserted along the small loop antenna wire as often as necessary. The loop antenna is broken up into loop segments, where each segment may or may not have a serial capacitor depending on the desired performance criteria. A loop segment may be one section of a single turn loop antenna, or one turn of a multiple turn loop antenna. Any number of loop segment resolutions can be implemented depending on the particular application. Each capacitor is selected so as to have a reactance that effectively cancels the inductive reactance of the loop segment preceding the corresponding serial capacitor. The advantage is that the instantaneous level of reactance on antenna stays nulled, and thus any reactive voltage difference between loop segments remains negligible, even with high current flowing inside the antenna. Moreover, the selected serial tuning capacitors are placed along the antenna wire to effect an average reactive voltage of substantially 0 volts across the antenna. The antenna is thus balanced about ground (GND).
The way that a loop antenna radiates power is not related to its voltage but to its current. In short, the reactive voltage on the antenna surface actually disturbs the electromagnetic radiation pattern more than it sustains it. Thus, an initial concern of an antenna matching technique should be to cancel the reactance of the antenna and thereby reduce the reactive voltage across the antenna. A low reactive antenna voltage translates to a reduction in the amount of antenna current escaping to external world grounds. A direct consequence of this reduction is a reduction in spurious capacitive radiation. In addition, the power at the self-resonating frequency of the antenna is increased as the overall spurious capacitance is reduced (i.e., antenna radiation is optimized because of maximum magnetic flux generation). Furthermore, the capacitive radiating antenna that is born from the capacitive leakage currents flowing to the surrounding environment grounds is inhibited because the electrical field in between loops is reduced. As a result, the overall ohmic loss of the antenna is reduced, particularly in antennas having multiple turn coils.
Adding too many capacitors is not practical even for loops printed on a PCB. There is a limit where the cumulative capacitance value becomes too large. Rather, the losses due to the equivalent series resistance (ESR) of added capacitors become significant. However, by carefully choosing the tuning capacitor values as well as the placement of each tuning capacitor within the antenna, the antenna will be balanced to ground and optimized for parasitic and ohmic losses reduction.
Thus, the present invention both balances the loop antenna to ground and reduces loop antenna parasitics by selectively placing tuning capacitors inside the coil of the small loop antenna. Parasitics such as ohmic losses, internal capacitive loss and capacitive loss to external world grounds are all reduced by the invention. The result is a highly versatile and reliable small loop antenna that has many applications including PCB applications in an electronically noisy environment. Under the principles of reciprocity, the present invention can be used to balance both transmitting and receiving antennas.